libstdc++: Define and use variable templates in <chrono>
Thi defines a variable template for the internal __is_duration helper
trait, defines a new __is_time_point_v variable template (to be used in
a subsequent commit), and adds explicit specializations of the standard
chrono::treat_as_floating_point trait for common types.
A fast path is added to chrono::duration_cast for the no-op case where
no conversion is needed.
Finally, some SFINAE constraints are simplified by using the
__enable_if_t alias, or by using variable templates.